Frequency references provided by oscillators are required in every clocked electronic system, including communication circuits, microprocessors, and signal processing circuits. Oscillators frequently consist of high performance piezoelectric crystals, such as quartz oscillators. The advantages of quartz oscillators are their stable operating frequency and high quality factor. However, the disadvantages of quartz oscillators are their relatively large size and unsuitability for high integration with electronic circuitry (e.g., CMOS circuits).
Based on these limitations of conventional oscillators, there is a strong interest in the development of fully integrated silicon oscillators. Integration is important not only for reduced size but also reduced power consumption. It is possible to realize an integrated silicon oscillator using the mechanical properties of silicon devices. For example, silicon microelectromechanical (MEMs) resonators can provide small form factor, ease of integration with conventional semiconductor fabrication techniques and high f•Q products. Accordingly, MEMs resonators are considered a desirable alternative to quartz resonators in real-time and other clock applications.
One example of a MEMs resonator includes lateral-mode piezoelectric resonators, such as thin-film piezoelectric-on-silicon (TPoS) resonators, which have been successfully incorporated in low-power and low-noise oscillators. FIG. 1A illustrates a conventional MEMs resonator 10 containing a resonator body 12 that is suspended opposite a recess in a substrate 16 by a pair of opposing tethers 14. This resonator body 12 may include a stack of multiple layers, including a semiconductor body, a bottom electrode on the semiconductor body, a piezoelectric layer on the bottom electrode and a plurality of top electrodes on the piezoelectric layer. The recess may be formed by selectively removing portions of a buried insulating layer within a semiconductor-on-insulator (SOI) substrate 16 containing a semiconductor device layer thereon. FIG. 1B is an SEM image of an 11th-order AlN-on-Si resonator fabricated on a 10 μm silicon-on-insulator (SOI) substrate. This resonator, which includes 11 interdigitated electrode fingers at a 20 μm pitch, includes three pairs of conventional beam supports (20 μm×2 μm) that operate to tether a suspended main body of the resonator to a surrounding substrate.
Other examples of these types of resonators are disclosed in U.S. Pat. No. 7,939,990 to Wang et al., entitled “Thin-Film Bulk Acoustic Resonators Having Perforated Bodies That Provide Reduced Susceptibility to Process-Induced Lateral Dimension Variations,” and in U.S. Pat. No. 7,888,843 to Ayazi et al., entitled “Thin-Film Piezoelectric-on-Insulator Resonators Having Perforated Resonator Bodies Therein,” the disclosures of which are hereby incorporated herein by reference.
Active frequency tuning techniques may also be utilized to improve MEMs resonator performance by, among other things, reducing the strength of spurious resonance modes that typically limit the quality factor (Q) of the resonator at a desired resonant frequency. Some examples of active frequency tuning in micromechanical resonators are disclosed in U.S. Pat. Nos. 7,639,105 and 7,843,284 to Ayazi et al., entitled “Lithographically-Defined Multi-Standard Multi-Frequency High-Q Tunable Micromechanical Resonators,” and in U.S. Pat. No. 7,924,119 to Ayazi et al., entitled Micromechanical Bulk Acoustic Mode Resonators Having Interdigitated Electrodes and Multiple Pairs of Anchor Supports,” and in U.S. Pat. No. 7,800,282 to Ayazi et al., entitled Single-Resonator Dual-Frequency Lateral-Extension Mode Piezoelectric Oscillators, and Operating Methods Thereof,” the disclosures of which are hereby incorporated herein by reference.
Unfortunately, while piezoelectric MEMs resonators may be interfaced with tuning and other oscillator electronics, such as transimpedance amplifiers, the appearance of sufficiently-strong low-Q spurious modes near the designed operation frequency of the resonator can cause degraded oscillator performance. For example, oscillator phase and gain conditions may cause a spurious mode to become preferred over a desired mode during start-up, causing an oscillator to lock into the spurious mode. In addition, feedthrough and coupling between the desired and spurious modes can degrade phase noise performance and increase frequency instability.